Not a good one. Dilation of the pupils can reflect not just the lighting conditions in a room but the thoughts and feelings of a person, mentally ill or not. For example, dilated pupils can indicate sexual arousal, as when making eyes at a comely Explainerette. Psychologists sometimes study the expansion and contraction of the pupils to find an objective measure of the mind’s inner processes, in something called pupillometry. This practice has been used to study everything from racial bias to sexual preference to what happens when the mind puzzles over a difficult math problem.

Dilation of the pupils can also be a side effect of using certain drugs. For example, LSD and mescaline both cause pupil dilation. Some people who suffer from mental illness may find themselves with dilated pupils caused by treatment with prescribed antipsychotic medications.

In the search for more objective, physiological signs of schizophrenia, other aspects of the eyes have also been studied, with mixed results. At least one study purported to find particular groups of facial features, including small eyes and narrow openings between the eyelids, in two subgroups of schizophrenics, but most psychiatrists don’t think such findings are useful. Researchers in the United Kingdom recently developed a computer model that could separate out a group of schizophrenics by tracking abnormalities in the movement of their eyes. However, so far the test isn’t accurate enough to be used for diagnosis.

March 3 2015 1:39 PMThe “Most Pleasurable Portrayal of Libertarianism“ Bonus SegmentDavid, Emily, and John discuss what Parks and Recreation got right about government.Emily Bazelon, David Plotz, and John Dickerson